1. Field of the Invention
Example embodiments of the present invention relate generally to a semiconductor device and method thereof, and more particularly to an image sensor and method thereof.
2. Description of the Related Art
Image sensors may be devices capable of converting optical images into electric signals. Image sensors may be deployed in a variety of device types, including but not limited to a digital camera, a camcorder, a personal computer system, a game machine, a security camera, a medical micro-camera, a robot, etc.
Conventional image sensors may be classified as charge coupled device (CCD) image sensors or complementary metal oxide semiconductor (CMOS) image sensors. CMOS image sensors may have a relatively simple operational mode and CMOS process technology may be compatible with a fabrication of the CMOS image sensor to reduce a fabrication cost and/or fabrication complexity. In an example, a conventional CMOS image sensor may include four transistors and a photodiode. In an alternative example, three transistors may be included in the CMOS image sensor.
A transistor structure of an image pixel may be fabricated through a CMOS fabrication process. In the CMOS image sensor, if a light is incident on the photodiode, the photodiode may absorb photo energy and may accumulate charges corresponding to a quantity or intensity of the incident light. A charge transfer unit may transfer the charges accumulated at the photodiode to a charge detector. An amplification unit may be combined with a substantially constant current source so as to serve as a source follower buffer amplifier. The amplification unit may output a voltage which may vary in response to an electric potential of the charge detector to a vertical signal line.
As image sensors are fabricated at reduces sizes, it may become more difficult for the charge transfer unit to transfer all of the accumulated charges stored in the photodiode to the charge detector. Charges remaining in the photodiode which may not be transferred to the charge detector may appear as an afterimage during a next read operation, thereby degrading a performance of the image sensor. Further, since the charges generated corresponding to the quantity or intensity of the light may be distributed among the charge detector as well as the photodiode (e.g., instead of a complete transfer), a conversion gain corresponding to the charges generated by a photoelectron may decrease. The remaining charges in the photodiode may also effectively reduce a charge accumulation capacity of the photodiode.